The machine tools to which the invention relates are utilized particularly in the aircraft building industry for contouring and drilling parts made of aluminium and composite materials, and in the automobile sector for producing master models, and models for bodywork, internal parts and dies.
Numerically controlled multi-axis machine tools of the conventional type in question are furnished with two longitudinal slide ways supporting a gantry or cross member capable of movement along a path parallel to the longitudinal axis of the slide ways. The slide ways are spaced apart in such a manner as to delimit a machining station between them, in which the work is positioned. The two opposite ends of the cross member rest on the two longitudinal guide ways. The cross member is traversable over the machining station and carries a machining head such as will clamp and drive a tool selected for the required drilling or milling operation.
As a general rule the machining head is traversable along the cross member in a direction normal to the direction of the motion induced in the cross member. Moreover, the head can be raised from and lowered toward the machining station and is rotatable about two axes, one vertical, the other horizontal, so that the tool can be angled relative to the workpiece. The work is placed in the machining station, and the machining head positioned on the cross member by the carriage in such a way as to address and machine the specified areas of the work. Positioning movements are induced by linear motors interposed between the various parts in relative motion and piloted by a programmable electronic controller.
Disadvantageously, machines of this familiar type present a structure made entirely of steel which, in the course of the various movements, absorbs the heat produced by the linear motors and by sliding friction. The exposure of parts to heat results in the deformation of these same parts. Deformation of the cross member is especially critical, since both ends of the cross member are coupled positively to the slide ways. In effect, as a result of the thermal expansion induced by heating and the restraining action of the couplings at each end, the cross member is caused to deflect and assume a bowed profile, impeded as it is, by the restraints, from elongating freely. The deformation affects the position of the machining head and jeopardizes the accuracy of machining operations.
The applicant finds that multi-axis machine tools of the prior art are improvable in a number of ways, and particularly as regards the dimensional stability of their structures in the face of temperature rises attributable to the various motors and to sliding friction.
The object of the present invention, accordingly, is to design a multi-axis machine tool substantially devoid of the above noted drawbacks.
In particular, it is an object of the invention to set forth a multi-axis machine tool in which machining accuracy is unaffected by the effects of heat on the machine.